Measuring devices are frequently applied in automation and process control technology, in order to ascertain a process variable, such as e.g. flow, fill level, pressure and temperature or some other physical and/or chemical process variable in a running process.
The travel-time method applied in fill-level measuring devices utilizes the physical law, according to which the travel distance equals the product of travel time and propagation velocity. In such case, the fill level measurement corresponds to the travel distance of twice the separation between a reference point of the transmitter and the surface of the fill substance. The wanted echo signal, thus the signal reflected on the surface of the fill substance, and its travel time are determined preferably based on the so-called echo curve, which shows in analog or digital form the amplitudes of the echo signals as a function of separation from the transmitter. The fill level value results then from the difference between the known separation of the reference point of the transmitter from the floor of the container and the separation of the surface of the fill substance from the reference point of the transmitter determined by the measuring.
All known methods can be applied, which enable determining relatively short distances by means of reflected measurement signals. If the measurement signals are microwaves, then both pulse radar as well as also frequency modulation continuous wave radar (FMCW radar) can be used. In the following, only the pulse travel time method will be discussed. However, the method of the invention is applicable likewise to other travel time measuring methods, such as e.g. FMCW. Microwave measuring devices, in the case of which pulsed measurement signals are freely radiated, are sold by Endress+Hauser, for example, under the mark ‘MICROPILOT’. Microwave measuring devices, which guide measurement signals along a conductive element into and from the container, are sold by Endress+Hauser under the mark ‘LEVELFLEX’. A device type, which works with ultrasonic signals, is sold by Endress+Hauser, for example, under the mark ‘PROSONIC’.
The transmitted measurement signals form with the received, wanted-echo signals a total measurement signal, which, in given cases, under actual measuring conditions, includes additional disturbance-echo signals. These disturbance echo signals have different causes, such as e.g.:                reflections on installed objects in the container and on the container itself        multipath propagation (retro reflection) and multimode propagation        dispersion of transmitted waves        foam and accretion formation by the medium filling and emptying procedures        reflection characteristics of the medium        noise        low dielectric constant of the medium        humidity in the container        unsteady medium surfaces.        
In the current state of the art, there are different approaches for removing these disturbance echo signals from the total measurement signal. Disturbance echo signals can complicate the evaluation and determining of fill level, when, for example, they cover the wanted echo signal.
Described in published international application, WO 03/016835 A1 is a method for evaluating measurement signals of a measuring device working according to the travel time principle, in the case of which a currently recorded measurement curve is compared with reference signal data. In comparing the reference signal data with the currently recorded measurement curve, there can be ascertained from time shifting of corresponding disturbance and wanted signals a correction factor, with which the sought fill level can be ascertained in the face of no present or evaluatable, wanted signal of the fill level.
The method and apparatuses disclosed in the above application for elimination of disturbance signals from the measurement signal all have the problem that they cannot react to changes of process conditions in the container, which influence the measurement signal, or to changes of the measuring method and measuring performance of the measuring device.
Described in German Patent, DE 10 2007 042 042 A1 is a method for ascertaining and monitoring fill level of a medium in a container by a field device, wherein the fill level is ascertained in the envelope curve from transmission signals and reflection signals based on travel times, respectively travel distances. The disturbance echo signals are stored in a masking curve, which is ascertained in a first measuring cycle from the envelope curve of the measurement signals in the empty container, and, for evaluating the envelope curve, an evaluation curve is stored, which is ascertained in a first measuring cycle from the envelope curve. In order to reduce the need for memory and so that the curves can be adapted to the measuring situation, the masking curve and/or evaluation curve are editable, in that the curve functions are formed from a reduced number of support points with appropriate, connectable functions.
Described in published German patent application DE 10 2005 003 152 A1 is a method for reviewing the orderly functioning of a fill-level measuring device, which works according to the travel time principle. For this, in predetermined or selectable time intervals, a measured value curve is ascertained at a current fill level; based on the current measured value curve, the wanted echo signal is determined. Then, the expectation value for the quality of the wanted echo signal is determined at at least one predetermined fill level based on the signal amplitude of the current wanted echo signal and based on an ideal echo curve ascertained under predetermined process and/or system dependent conditions. The ascertained expectation value for the quality of the wanted echo signal at the predetermined fill level is compared with a predetermined critical value for the quality, and an error state is diagnosed, when the ascertained expectation value subceeds the critical value for the quality.
An object of the invention is to provide an improved and more accurate method for evaluating measurement signals of fill-level measuring devices.